Cellulose acetate solution



Jan 3, 1.950 c. R. FORDYCE ET AL 2,492,977

CELLULOSE ACETATE SOLUTION Filed Nov. 5, '1946, 2 sneek-sheet 1 TIME/M/NU'ES? 0N CAST/N6 SURF/ICE AT 707.70807.:

B @What/714. /mw

A TTORN S Jan. 3, 1950 c. R. FORDYCE l-:T AL

CELLULOSE ACETATE SOLUTION 2 Sheets-Sheet 2 Filed Nov. 5, 41946 FOR A CONSTA/Y T CUPED TH/CKNESS 0F. 0055 VC/#55` TEMPERA ru/QE ("f-T) ATTORI Patented Jan. 3, 195@ CELLULOSE ACETATE SOLUTION Charles R. Fordyce and Bruce E. Gramkee, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application November 5, 1946, Serial No. 707,807

s Claims.

This invention relates to a cellulose acetate solution.

For many years cellulose nitrate has occupied a unique position as a material for the production of commercial motion picture film base. Since the advent of commercial cinematography some forty years ago, cellulose nitrate has remained preeminent as a base for motion picture film by virtue of the fact that it displays a peculiar combination of high tensile strength, flexibility, moisture resistance and rigidity. High tensile strength is necessary to give the lm the wearing qualities which will enable it to resist the tearing and abrading action which occurs at the perforations when the film is passed over the sprockets of processing and projection machines. Flexibility is essential to proper behavior on handling. Good moisture resistance is necessary to prevent excessive curl under conditions of varying humidity, to prevent the development of waviness which would otherwise result from variations in the moisture content, and to reduce the dimensional change which the photographic film undergoes in the drying operation following development of the photographic image. Good rigidity, not only reduces the tendency of the photographic nlm to curl, .but also helps prevent unsteadiness during projection.

All of these desirable qualities are possessed to a high degree by cellulose nitrate film. Cellulose nitrate is, however, an inorganic acid ester of cellulose and film made therefrom has the drawback of infiammability and instability, the film `becoming discolored and .brittle on long standing unless the cellulose nitrate has been manufactured with the greatest of care and skill.

In the entirely separate and distinct field of organic acid esters of cellulose, many such esters have been suggested for use in producing commercial motion picture film base; of these esters, cellulose acetate and related cellulose mixed esters, such as cellulose acetate propionate and cellulose acetate butyrate have thus far proved to be the most useful. However, those esters which are susceptible of casting into films from acetone, although quite satisfactory for use in the production of cut sheet, X-ray amateur cine and other types of films in the use of which safety is the paramount factor desired, nevertheless do not give to films the physical properties enumerated above so as to permit their use for this purpose Vin the commercial motion picture field. For example, films produced from acetone-soluble cellulose acetate have low tensile strength, poor dimensional stability and a lower degree of rigidity i 2 than cellulose .nitrate lms. Moisture susceptibility is a particular defect of such cellulose acetate films -giving rise to a. dimensional change, as shown by swell and.y shrink measurements, of about twice that desired. Also, commercial motion picture films vary in swell and shrink amplitude from about 0.5% to 0.8%, whereas the cellulose acetate safety films available up to the present time vary in swell and shrink amplitude from 1% to often as high as 1.75% and have been for this reason, among others, unsuitable for professional motion picture use. Similarly, lms produced from the mixed esters, although in some cases equal to commercial motion picture films in dimensional stability, are inferior in tensile strength and rigidity.

Among attempts to provide a satisfactory cellulose organic acid ester eine lm base many investigators have thought that fully esteried cellulose (cellulose triacetate) would have potential value for this purpose if some means could be devised for satisfactorily dealing with it in the casting or film-forming operation.

While lms produced from cellulose triacetate have moisture resistance which is superior to that of films produced from acetone-soluble cellulose acetate, and a higher tensile strength, the material has very limited solubility and has presented extreme technical diiculties in the casting operation. Methylene chloride, and more particularly mixtures of methylene chloride with methyl or ethyl alcohol, have for many years been about the only useful active solvents for cellulose triacetate and even today are the only practical basic solvents adapted for commercial use. It has :been found, however, that films cast under the usual conditions heretofore known to the art, from solutions of cellulose triacetate in these solvent mixtures, have certain undesirable characteristics, such as brittleness, which make them of inferior value in the commercial motion picture industry.

Fully as important as the above-mentioned deciencies of known cellulose triacetate film-forming compositions is the fact that there is an economic disadvantage in their use, because of the much lower speeds heretofore attainable in the casting operation.

We have discovered that cellulose acetate having an acetyl content of from 42.5% to 44.0% will, when formed into a sheet in the manner hereinafter described, give a sheet which has properties admirably suited for photographic film base of a character fully as acceptable and even in some respects superior to present day commercial motion picture film base.

An object is to provide compositions from highacetyl cellulose acetates adapted for the casting of film base having such physical properties as to render it acceptable for use in the commercial motion picture industry. Still vanother object is to provide from high-acetyl cellulose acetates, compositions adapted for the casting at high speeds of cine film base having high tensile strength, good flexibility, moistureresistance and rigidity, non-infiammability and good stability.

These objects are accomplished =.by the following invention which is based ii'undamentallyupon the very unsusual discovery fthat solutions of-cer tain high-acetyl cellulose acetates in menthylene chloride-alcohol may be converted intosolutions which are capable of being cast by our process -into lm base having physical properties Ahighly desirable for commerciallmotion picture film base, byadding aspecial diluent thereto. Specifically wehavefoundithat if a cellulose acetate contain- .ing from 42.5% ;to144.0% acetyl ,and preferably 43.0% `to 43;5% acetyl :is :dissolved in a solvent composed of 465% Zto 90% by `weight methylene chloridafrom 5% to 15%.methyl alcoholor ethyl alcohol, and, .as a special ingredient, 5% to 20%, Vbasedon the weightof the total solvent, or proipylene 1chloride,the resulting solution may be cast and'strippedfrom thecastingsurface under certainspecialconditionswhich vwill result in afilm yhaving Especial .properties .highly desirable for Hereintate as used lin our invention, -we refer to cellulose acetate having an acetyl content .of from-42.5% ,to 44.0

As to the special lconditions, our film-forming solution is deposited'upona nlm-,forming surface Vwhich ismaintained within theipracticallimits of Y50 F. to 190 F. andisallowed to remain-on such `film-forming surface onlyzuntiLthe lm has-cured down to 125% to 50% '.greater'than the thickness which .the `i`1lmwill have -when completely cured,

-whereupon the Aillrn 4is stripped .from the Vlmcastingsurface and subjected `to further curing in the usual mannerknownto the'art.

The unusual.andvdistinctiveproperty of propylene chloride by `.virtue of `which it converts the ymethylene chloride-alcohol solvent combination Yto anunusually effective casting'solvent for highacetylcellulose acetate is brought into sharp relief when one takes into .consideration the fact that closelyrelated solvents such as butylene chloride as well assuchsolvents as acetone,.methyl ethyl ketone, .benzene and many others, have no such effect when added-to=methylene chloride-alcohol, .and films-.cast from such solvent .combinations do not have the required physical properties. We have also discovered that ethylene chloride ,has an eect similar'to that of rpropylene chloride Abut requires a different range of proportions and different range of stripping thickness from that of the propylene chloride compositions, .as vdescribed and claimed in our application of even date, Serial No. 707,808.

`In the following examples and description we have set forth several of the preferred embodiments of our invention, but they are included merely for purposes of illustration and not as a .limitation thereof.

In .the accompanyingdrawings:

Fig. 1 andFig. 2 are graphic representations of `certain ofthe .conditions under which our process is operated.

Typical film-forming operations caried out in accordance with our invention are given in the following examples:

Example 1 100 parts by weight of cellulose acetate of 43.0% acetyl content was Adissolved in a'mixture of 4.80 parts by weight `oimethylene chloride and parts of methyl alcohol. After the cellulose sester had completely dissolved, 60 parts of propylene chloride .and l5 parts of triphenyl phosphate were added. The mixture was stirred until completey .homogeneous .and then allowed to stand -.unti1 .free kfrom bubbles.

The solution .was maintained at F. and fed into the hopper of a lm casting machine from whichit'was applied in uniform thickness to the surface of a rotating casting Wheel at a temperatureof "75 F. in such a thickness as to obtain a film of .0055 inch `thickness upon complete evaporation :of Athe fsolvent. After .a :period of six minutes upon the casting surface the film was stripped therefrom. The thickness .at stripping '.waslfound to ;b'e.;0085, this thickness being-.ap- :proximately 55% greater Ithan :the cured thickfness. .The film .was then carried through aseries :of suitable curing sections `to evaporate residual solvent vandtth'en .wound :into .1a.roll. The product `was found to bea clear, transparent'filmshowing -a1flexibility of 12-16 folds land a tensile strength 1of122.4 kilograms (115;100 pounds :per square inch). 'The Vdimensional vswell and shrink wasfound to lbei0.'75% in the lengthwise direction and 0.88% 'inlthe widthwise direction. This example falls -within=areaB Vofithe graphslof Figs. 1 and 2.

As a comparison, similar runs under `similar -con'ditionsltoExample l involving use of the same v'cellulose ester dissolved in mixtures of :methylene 'chloride and methyl alcohol without the addition Vof altliird-component'were' made, 'but in this case the "film could `notbe Ystripped from the casting 'surface until it was iless than v50% greater in "thickness vthan the cured thickness and gave a 'considerably morefbrittle-lm and one with much 'greater swell and shrink 4amplitude than that produced accordingito Example 1. Similarly, the use of ethylene chloride in place of propylene chloride, under similar conditions to Example 1, resulted'in lms-of much'less-dimensional stabil- `ity. The exceptional individuality of propylene vchloride 'in ythe instant invention as compared With ethylene chloride as an ingredient of the solvent formula, lor with Amethylene chloride-a1- colrol 'without such a diluent, all under similar conditions, is shown in the following table:

Example 2 Alm-casting solution composed of parts of cellulose acetate of 43.8% acetyl contentand-430 parts.o`f.methylene chloride andO parts of methyl '75 alcohol wasprepared as'in'thepreceding example and there was then added 120 parts of propylene chloride and 25 parts of triphenyl phosphate. Film was continuously cast from this solution by depositing it at 80 F. on the surface of a rotating casting wheel maintained at 75 F., and in such thickness as to result in a completely cured film .0055 thick. After 6% minutes on the casting surface the iilm was stripped therefrom and found at stripping to be .0083 thick, this thickness being approximately 51% greater than the cured thickness. After complete curing the resultingA film was a clear, strong, transparent and flexible product having a flexibility of 17-18 folds, a tensile strength of 22.1 kilograms (14,900 pounds per square inch), and a swell and shrink amplitude of 0.61% in the lengthwise direction and 0.75% in the widthwise direction. This example falls Within area B of the graph of Figs. 1 and 2.

Eample 3 100 parts of a cellulose acetate of 44.0% acetyl content was dissolved in a mixture of 680 parts of methylene chloride and 80 parts of methyl alcohol after which there was added 40 parts of propylene chloride and 10 parts of triphenyl phosphate. The solution at 80 F. was continuously cast into a film using a moving casting surface maintained at a temperature of 70 F. and under such conditions as to produce a fllm .0055" thick. The film Was removed from the casting surface after a period of minutes in which time it was found to have a thickness of .0090 thick, this thickness being approximately 64% greater than the cured thickness. After complete curing a clear, transparent, flexible film of high tensile strength was obtained which had a swell and shrink amplitude of .75% in the lengthwise direction and .85% in the widthwise direction.

This example falls within area B of the graph of Figs. 1 and 2.

Example 4 A cellulose acetate solution prepared identically p to that in Example 1 was cast at 80 F. onto a moving casting surface also maintained at 80 F. in such an amount as to produce a completely cured lm of .0056 thickness. After three minutes on the casting surface this lm was continuously removed under which condition it was found to have a thickness of .012, this thickness being approximately 114% greater than the cured thickness. After evaporation of the residual solvent the product was found to have a tensile strength of 23 kilograms (15,500 pounds per square inch), a flexibility of 18 folds and a swell and shrink amplitude of 0.62% in the lengthwise direction and 0.89% in the widthwise direction. It is to be noted that in this example the lm was cast at a speed twice that at which the film of Example 1 was cast, thus falling Within high speed operating areas D of the graph of Figs. 1 and 2.

Example 5 100 parts by Weight of cellulose acetate of 43.0% acetyl content Was dissolved in a mixture of 480 parts of methylene chloride and 60 parts of ethyl alcohol. After the cellulose ester had completely dissolved, 6.0 parts of propylene chloride and 15 parts of triphenyl Vphosphate were added. The mixture was stirred until completely homogeneous and then allowed to stand until free from bubbles.

The solution was maintained at 80 F. and fed into the hopper of a film-casting machine from which it was applied in uniform thickness to the moving surface of a casting wheel main-- tained at a temperature of 75 F. in such a thickness as to obtain a film of .0055 upon complete evaporation of the solvent. After a periodv of six minutes upon thecasting surface the film was removed therefrom. The thickness at this time was found to be .0090, this thickness being ap-v proximately 64% thicker than the cured thickness. The lm was carried through suitable cur-A ing sections to evaporate remaining solvent and then wound into a roll. The product was found,

to be a clear, transparent lm of good flexibility and strength. This example falls Within area B of the graph of Figs. 1 and 2.

Example 6 A cellulose acetate solution prepared identically to that in Example 1 was cast at `80" F. onto a casting surface maintained at 50 F. in such an amount as to produce a completely Icured iilm of .0055 thickness. After 41/2 minutes on the casting surface this lm was continuously removed under which condition it was found to have a thickness of .011, this thickness being 100% greater than the cured thickness. After evaporation of the residual solvent the product was found to have a tensile strength of 23 kilograms, (15,500 pounds per square inch), a ilexibility of 16 folds and a swell and shrink amplitude of .75% in the lengthwise direction and .95% in the widthwise direction. The use of a low temperature casting surface in this operation resulted in coagulation of the cast film in such form that it could be successfully removed from the casting surface at a speed of 41/2 minutes, whereas a warmer casting surface (70 F. to 90 F.) would have required either a slower or faster stripping speed to avoid undesirable sticking.v

`This example falls within the lower right hand "portion of area D of Fig. 2.

While the acetone-insoluble cellulose acetates having an acetyl range of 42.5% to 44.0% are all operable in producing lms of improved physical properties in accordance with our invention, we prefer to employ the acetates having an acetyl content Within the range of 43.0% to 43.5% because they provide products for which film cements can be more readily prepared and applied than is the case with films produced from cellulose acetates of higher acetyl content and because cellulose acetates of 43.0% to 43.5% acetyl content are better adapted to the casting and curing conditions of our invention.

The cellulose acetate best suited for our invention has a viscosity at 25 C. 0f from 3,000 to 10,000 centipoises as determined in a 10% solution by weight in a solvent composed of methylene chloride and 10% anhydrous methyl alcohol by weight.

The ratio of cellulose acetate to solvent mixture most useful in our invention is approximately 1 to 5 although ratios up to 1 to 8 can be employed.

It will be apparent from the above examples and description of our process that it has many advantageous features which distinguish it from the prior art. First and most important is the fact that by an extremely simple expedient, namely, the addition of propylene chloride to the methylene chloride-alcohol primary solvent, we have solved a problem of llong standing in the motion picture industry, the problem of how to produce a safety type film which would have physical properties equal to or better than present-day commercial motion picture film so as' to etranger?? permit itsi` ready ands convenient'. use fini modern processing` and:v projection apparatus.

Specifically,v our.' invention has,:. asi indicated; above;- resultedzin the=provision;of a high'- acetyl cellulose. acetate'.ilmahavingfphysical properties inf: all important; respects. comparable.. to.those ofrthef.best commercial.zmotionpictureliilm;v This willi be apparent? from' the'r following tabulationv ofv data-1 obtained'by testingxrepresentative pres-sv entday `commercial safetyfcine film, commercial' motionlpictureiliilm'` and the film' prepared.` in accordance with thefinstan't' invention. At 'thisf point itmighttb'e: well vtofpointout that commer'-V cial safety cine lms have been used Lprior to the` advent of our invention only for so-called amateur movies. Such illm is normally made in 162mm; width and'usedinilengthsof from 5'0f to 4'00fet. Such illmsarefmade'ffrom theacetone-Y soluble-f type -of cellulose acetate'` (approximately 39141 acetyl) theY mixed'esters` suchias. cellulose acetate propionate and cellulose acetate butyra-te are also used forsuch amateur cine lm. All ofthese organic acid' esters ofv cellulose inv stieetffrmhave suilicient'tensile strength, suili-y cien-tly lowswell 'and shrinkamplitude; and other properties; to-be very useful for amateur pur-- poses for' the' reasonVV that such iilml is usually projected: onlyV infrequently and isgenerally handledwith acertainamount of care and con-A- sidration. However, as shown byv theiollowing` tabulation, the propertiesfoffsuch safety film areV not such asA willstand the rigorsiof repeatedpro f jection in the average movie theater, several` times a day andfor long runs of possibly Weeks in succession und-er conditions typified bythe well-known' slogan The show' must gok on;'

from the sheet material.

1 temperature tl'iermostatically4 controlled. water' bath at' 100o4 F.' and" left for 17 hours. The. samplesarethen wiped to remove excess moisture and reconditioned'at'50% relative humidity and A measured again and the dimensional change computed.- This. test.` measures the irreversible shrinkage, if any; due to lossoifsolvent-from-the lm^material and also that due-tor the-release of. internal.mecl'ianical strains.-

Thel samples are then placed in a waterxbath at:

'A 125 F; forr30 minutes, then removed and meas-- ured'xfor llength as speedilyfas possible after re^ Inovingsurplus Water. Ther samplesratre4 thent placed inan oven at12'5f F. for onehour` then:

. f taken'out and-measured. Thisv'cycle is repeated' three times or untilthe difference between the wet' and' dryreadings becomesv constant. The difference between the last wet and dry freadings in percentage is the per cent swell andshrinle amplitude;A This'test measures the permanent', characteristicV tendency ofthe material to' swell High-Acetyl Y Commercial Commercial Cellulose Safety Cine Motion Acetate Film Films Picture Films Base of Our Invention I Tensile-Strength; p. s. i l', OOO-12,100 13, 500-17, 000 r1Zl,-5()0e16,000` Flexibility (SchopperioldsL. i 5-15 15-20 y 1520 Modulus of Elasticity (tension s p.` s..i 3.5 5.' 56.'5 4. 8f5. 8- Swell und Shrink Amplitude (L'engthwise Direction) 1. 00-2. 00 OJO-0. 80 0. 60-0. 90 i Swell 1n Water at 70F'. (100 minutes); --per cent.; 0.80-11 50A 0. Ztl-0. 40 0. 30-0. 40

Swell andi'shrink amplitude and swell in water? at70 F. are-'alternate tests showing thesame property in the iilm base stock; i; e; dimensional f' stability-with moisture'.` All' of'the aboveve' tests were made inthe lengthwise direction for the reason that, in manufacture; motion* picture' lmsuch"as tha-i735 mm. wide orI I6 mm. Wide, is`preparedby slitting lengthwise'al mucli wider' sheet, and for the further reason that, due? to its greatv length compared to width and the fact' that it travels through' a camera; processing` machine and projector in the lengthwise'direc tion, good strength and 10W swell and shrink amplitude in -the lengthwise' direction is-r the more important; Under thecasting, stripping' and curing conditions of our invention, the-above' properties in-the widthwise1direction will more orless approximate thoseinthe lengthwise direc-'-Y tion: but are not'of great consequence; hence wen lia-ve notA included them 1inthe above tabulation; some' of these properties inthe'widthwlse direc tionare given in the; examplesias a matter off interest and comparison;

The swell and shrink. amplitude'. test is conducted'. as follows: Samples off iilr'n4 or' sheeting; to; be testedy are conditioned 'and'measured".in` a; constant humidity` room. at. a relativev humidity` of" andi at: a dry: bulb; thermometer reading andshrinlcunder, the inuence of absorbedand desorbedmoisture.

From the. abovetabulation the comparability. of the physical properties of film produced in accordance Withour invention andthe properties of. present-day` commercial. motion picture film, will be apparent; ladd to these: propertiesthe noniniiammabilityA and stability of the lm oi our.

invention and it is easy' to perceive the. superiority thereof.` Also: apparent is the improx/ement in our films over the present-.daycominercia/l safety cine films, particularlyin the. mat ter otdimensionalstability asv indicated bytheir: very low swell and shrink amplitude.` It; isto be noted that the filmsotthe instant invention have a swell and shrink' amplitude only about one-'half that-oicommercial'safety iilins and also that their; flexibility is much `greater so that the lin'base of our-inventionis-virtuallya diierent`I kind of l'iilmb'ase than previous safety i'llm.

However, of i all the' features of Vour invention, the most unusual and important'is the fact thatl theIm-forming solutions are so' designed that the film is stripped -from thecastingsurface While it is still 50% to 125% thicker thanit' is to lie-- comelaiter it is completely'cured. Infact, it appears thatf the propylene chloride ingredientA oi' the solventcombination actually.- contributes the vv'peculiar `and highly individual characteristics of the film-forming solution which permit it to be cast` and stripped under these conditions by bringing about a state of coagulation in the partially cured film which at certain specific stages permits the film readily to be stripped from the coating surface to produce films of satisfactory physical properties. This most unusual aspect of our invention will be more apparent by reference to the curves of Fig. 1, which illustrate the -relation of stripping thickness to casting speed when the casting surface is maintained at 70 F. to 80 F.

In every film-making operation primary consideration must be given to the matter of the ultimate or cured thickness of the film. The upper set of curves of Fig. 1 illustrates the relation between the cured lm thickness in inches and the film-forming speed when high-acetyl cellulose acetate lms are produced in accordance with our invention, speed in this case being expressed in terms of time in minutes during which the film-forming solution has remained on the casting surface, that is, the time which elapses from the instant the solution is deposited on the 'casting surface to the instant at which it is 'stripped therefrom. Time on the casting surface is generally referred to as stripping time. Thus, in this set of curves cured thickness is plotted against stripping time. The lower set of curves, on the other hand, illustrates the relation between the thickness of the film at stripping while it contains a relatively large amount of solvent, and the time in minutes during which it has remained on the casting surface. Thus, in this second set of curves the uncured thickness of the film is plotted against stripping time.

It will be observed that both curves conform to the same general pattern, but that the respective areas delimited-by the upper set of curves are smaller than the corresponding areas of the lower set of curves. The difference in the extent of the'respective areas of the two sets of curves is, of course, explained by the fact that in the one vcase conditions are illustrated in which the film is substantially completely cured, while in the other case the film contains appreciable amounts of residual solvent.

Referring to the upper set of curves, the area A represents a range of film-forming operations in which the resulting film will be comparatively well cured at the time of stripping, that is, an area representing casting conditions in which, for a given cured thickness, the film has remained on the film-forming surface for such a length of time as to permit removal of a very substantial percentage of the original solvent. This area, therefore, represents such a slow film-forming operation and conditions in which films of such inferior physical characteristics are produced, as to lie outside the scope of our invention. Area B, on the other hand, represents a range of operation in which, for the same given cured thickness, the film at the time of stripping from the casting surface is approximately 50% to 125% Area C represents another range of operation or set of yconditions in which, for the same given cured film thickness, the casting speeds are greater than those falling within the conditions of area B. Under such conditions it has been found that, if for the same given cured thickness of lm, the film-forming speed is increased, that is, the time on the casting surface is decreased, a point is reached where the lm will not strip properly but tends to adhere tightly to, and to leave deposits of film material on, the casting surface. This point is reached at the boundary between area B and area C. As the speed is still further increased a point is reached where the lm will again begin to strip properly. This point lies on the boundary between areas C and D. Continuing to assume the same given lm thickness, as one further decreases the time during which the film remains on the casting surface, that is, increases the film-forming speed, a point is finally arrived at where any further increase in the film-forming speed results in imperfect stripping and a fllm of unsatisfactory physical properties. This point is reached at the boundary between areas D and E.

The above-described phenomena will be more fully illustrated by reference to the conditions prevailing when coating a film of a given finally cured thickness, which is Within the range of standard cine lm, namely, a range of .005 to .006. For example, assuming that the film is to have a cured thickness of .006" and also to pos- Sess the desired physical properties of high ten- Sile strength, flexibility and good dimensional stability, it is apparent from the upper set of curves of Fig. 1 that such a film could be cast at two different speeds. By following from right to left along the abscissa corresponding to a cured thickness of .006, it will be observed that the operative range of film-forming speed to obtain a lm of satisfactory physical properties begins at the point where the abscissa intersects the lowermost curve, that is, at a point corresponding to a stripping time of approximately 'l1/2 minutes and end-s at the point where the abscissa intersects the second curve, that is, at a point corresponding to a stripping time of approximately 5%, minutes.

However, by reducing the stripping time below 5% minutes one runs into the inoperative area C until the stripping time is further reduced to about 4 minutes at which point the lm again begins to strip properly and to have good physical properties. The speed may then be further increased until a minimum stripping time of 3 minutes is attained, at which time poor stripping and unsatisfactory physical properties again begin to appear.

It will thus be seen that our film-forming solutions are susceptible of satisfactory casting at two distinct speeds, one roughly twice the other. This is a most unusual phenomenon and one which has not been observedY with any other film-forming compositions.

We have referred to the fact that, in accordance with our invention, to produce a high-acetyl cellulose acetate lm having the desired physical properties the film must be stripped from the nlm-forming surface while itY is 50% to 125% thicker than it is to become when cured. This is illustrated graphically by a comparison between the upper and lower sets of curves of Fig. l. For example, assuming the same cured thickness as above, namely .006, if one follows down the ordinate from the point where the .006"

aratro??? the film-forming speedto the maximum for'the productionofthis sameiultimatecured thickness, it: will be. observedthat` this willbe attained at theV pointY whereY the .006 abscissaV intersects the boundary betweenareas D' andV Ei at the point b of the upperset. of. curves. If.' onev'follows; down the ordinate from this point to theA point Where the ordinate.- intersects the; correspondingY curve of' the'. lower. set: at the'. point b', the stripping thickness will be foundgto beiabout .0135", this thickness thus being approximately 125% of: the

cured thickness.

If the temperature ofthe casting surfaceA is lowered, somewhat different stripping characteristics result, asA shown in the-v curves of. Fig; 2, which. illustratesoperating conditions for producing a product of approximately .0055 cured thickness. The shaded area' Arepresents slow strippingzspeeds, in Whichthe lmisnot stripped in a. c'oagulated conditionA as; described by our process and'. does not, therefore, result. in good physical properties. Theclear. area B`2represents the operative range'. in which. a. strippable sheet istV formed, which; maybe; remoyede fromthe casting surface within a stripping, thickness; range of .0033" to .0125. or; approximatelyY 50%- tos- 125 greater than the curedthickness. The: shaded areaV C represents; the range in: which'. the; partiallyv cured film adheres: to.l theA castingV surface and cannot be strippedrsuccessfully.. The. upper clear area-.D representsa higher speedrange of operatiom in which' the film isin: the form of a strippablea sheet; which:v does not: stickto thes casting surface and can be stripped while- 50% to 125% thicker; than itis to, become` when completely cured. The; shaded area Errepresents an f insuiicient degre'eofV curing to permit stripping.

While' present cinelmi base' stocku has amore orsless.standardthickness ofi.005" to..0061, it will beV seen fronrthe'` curvesor graphs of Fig; 1 that our invention is applicable tohigh-acetyl cellulose acetatebase stock. ranging in thickness from .004 to'.010 and, inthe claims, safety cinelm base stock will be understood to refer to-this range of.- thickness;

InFig; 2.We have shown acasting surface temperature range'of from 50 F. to 90 F., andthiS-is thev practicalV operatingrange of; casting, surface temperature-for-our invention. If. one=maintans the film-casting surface at a temperature much in excess of: 90' evaporation. takes placewith such .rapidity thatbubbleswill form inthe acetate sheet: and. thus givea. lmbase: stock whichwould not be useable; On the,A other hand; ifL one attempts to operate our invention with a film.- castingf surface maintained. at a temperature muchV below 50 F., the curing time before-stripping can be accomplished increases to apoint 12 whererlittleornothingis gained by` attempteduse of the invention. Accordingly, wehave.- establlished thelimitsof50 to 90 F.. asrtherv practical limits within whichA to;` maintain the hlm-casting surface.

It will be: understood, ofcourse, thatthe graph constituting Eig. 2 isv drawnfor a flnel cured .thicknessof .0055l for the-acetate sheetand thatthis graph is constructed. to showfvariation: in` strippingtime resulting from variation inftemperature ofatleffilm-casting surface; from F. tol 90 F. for: thisV particular: nal thickness. of: acetate sheet.

What we claimisz.

1. Asa new composition of' matter; a cellulose lacetate film-forming solution capable of being cast. in the form of a sheet attwo distinct ranges of nlm-forming speed-for a givencured lm thickness, said: solution being composedAv of cellulose acetate having an acetyl content within theirange of 42.5% to 44.0% dissolved in a solvent mixture composed'of:65.% to: 90 %by. weight of' methylene chloride, 15 to* 5 of .anxunsubstituted aliphatic monohydric alcohol ofi 1 to: 2; carbon atoms. and 20 to 5% propyleneichloride;

2. Asa new'composition.ofmatter; a. cellulose acetate nlm-forming. solution capable of. being cast in the formof aisheet attwo distinct ranges of lm-formingspeedafora giverncured-fllm thickness, said solution being.- composed of. cellulose acetate having an acetyl contentwithin the range or 43.0% to'43i5'% dissolvediin asolvent mixture composed of to9.0.%.byfweight of methylene chloride, 15% to 5% of` an unsubstitutedaliphatic monohydric alcohol ofA 1. to 2 carbon. atoms: and 20% to 5% propylenee chloride:

3. Asa newA composition of matter, a cellulose acetate film-forming solution7 capable of being cast'in thetform` of asheet at'two distinct ranges offlm-formingspeed'for-a given cured film thickness, saidv solution. being composed of cellulose acetate having'an acetylicontent within the range of 43.0% to43.5%` dl'ssolvedrin a; solvent4 mixture composed. of %V by Weight, of' methylenek chlo- REFERENCES CITED The following referencesY are of record' in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,384,188 Goemer July'12, 1921 1,668,946 Clarket al May 8, 1928 1,844,365v Moss Feb; 9, 1932 1,880,067 Barthelemy4 Sept. 27, 1932 1,880,466 Noller Oct. 4, 1932 1,896,145 Stand et al Feb. '7, 1933 1,930,134 Smith Oct. 10, 1933 1,969,183 Reid Aug. 7, 1934 2,000,622 Taylor May 7, 1935 2,037,292 Weihe Apr. 14', 1936 2,150,250 Scott Mar. 1'4, 1939 2,232,012 Rooney Feb; 18, 1941 2,319,052v Fordyce May 11, 1943 FOREIGN PATENTS Number Country Date 512,967' Great Britain .Sept. 29, 1939 

1. AS A NEW COMPOSITION OF MATTER, A CELLULOSE ACETATE FILM-FORMING SOLUTION CAPABLE OF BEING CAST IN THE FORM OF A SHEET AT TWO DISTINCT RANGES OF FILM-FORMING SPEED FOR A GIVEN CURED FILM THICKNESS, SAID SOLUTION BEING COMPOSED OF CELLULOSE ACETATE HAVING AN ACETYL CONTENT WITHIN THE RANGE OF 42.5% TO 44.0% DISSOLVED IN A SOLVENT MIXTURE COMPOSED OF 65% TO 90% BY WEIGHT OF METHYLENE CHLORIDE, 15% TO 5% OF AN UNSUBSTITUTED ALIPHATIC MONOHYDRIC ALCOHOL OF 1 TO 2 CARBON ATOMS AND 20% TO 5% PROPYLENE CHLORIDE. 